1. Field of Invention
This invention relates to electrodes, electrochemical cells incorporating said electrodes, compositions of matter related to said electrodes, a process for producing said electrodes, and a process for making a previously unavailable material needed to make said electrodes.
2. Related Patent Applications
A water purification device utilizing the electrodes described herein is provided in the related U.S. patent application Ser. No. 07/975,059, filed Nov. 12, 1992. The above identified related application also claims the use of these electrodes to produce hydroxyl free radical, and to degrade by oxidation chemical substances dissolved in water, and provides examples. The above identified related application is hereby incorporated by reference.
3. Discussion of Prior Art
The present invention provides an electrode that produces hydroxyl free radicals when it is polarized to a sufficiently large anodic potential. Hydroxyl radical is a very powerful, nonspecific oxidizing species which attacks most organic molecules. Hydroxyl is produced by irradiation of particles of titanium dioxide dispersed in water with ultraviolet light (Kormann and others 1991), and hydroxyl thus produced reacts with and degrades organic substances in solution. The photochemical process remains largely a laboratory curiosity, because sunlight includes only a small fraction of usable UV energy, and the photochemical process has a small quantum yield.
The present invention provides electrodes wherein the base metal is titanium or a titanium alloy, covered with a suitably doped coating of titanium dioxide. While titanium is used as a substrate for coated anodes in several applications, pure titanium dioxide has very low electrical conductivity. The anodic oxide layer spontaneously produced when titanium metal is anodically polarized does not make a useful electrode, because the current density is low to begin with, and decreases rapidly with time as the thickness of the oxide layer grows.
Titanium anodes are used in several applications wherein the titanium metal substrate is coated with an adherent layer of titanium dioxide which includes admixtures which increase its electrical conductivity. For example, titanium anodes coated with titanium dioxide layer that is doped with platinum group metals are commonly used in cathodic protection systems. Titanium anodes coated with titanium dioxide including a large fraction of ruthenium dioxide and other metal oxides (commonly tin dioxide) are used in the electrolytic production of chlorine. Aside from their large content of precious metals, these electrodes are ill-suited for generating hydroxyl for a fundamental electrochemical reason: other anodic reactions occur at potentials much less positive than the values required to generate hydroxyl, and these competing reactions prevent the potential from rising high enough to generate hydroxyl. In particular, titanium anodes doped with platinum group metals are very good oxygen electrodes, and efficiently oxidize water to oxygen far below the positive potential required to generate hydroxyl. Ruthenium-doped anodes oxidize chloride ion to chlorine gas at near the equilibrium potential for that reaction. Also, ruthenium doped anodes are severely corroded when operated at potentials significantly more positive than that required to generate chlorine, particularly at pH above seven. For these reasons, prior art coated titanium anodes are completely unsuited to electrochemical generation of hydroxyl and the nonselective oxidation of chemical substances dissolved in water.
U.S. Pat. No. 3,846,273 and U.S. Pat. No. 4,484,999 each describes an electrode that includes an oxide coating which may include the oxides of Ti, Nb, and Ta, and an electrode preparation method which involves brushing onto a titanium metal substrate, a solution which includes compounds of the metals desired in the oxide coating, and then heating the electrode in air to evaporate and thermally decompose the coating solution and produce the oxide coating. However, neither U.S. Pat. No. 3,846,273 nor U.S. Pat. No. 4,484,999 hint or suggest that a +4 valence state is desirable in electrode manufacture. Indeed, their teachings are directly to the contrary. Heating the electrodes in air produces an oxide coating wherein Nb and Ta are in the of +5 valence state, a state that is vastly inferior in electrical conductivity to the +4 valence state of Nb or Ta specified by us. Furthermore, the electrodes described by U.S. Pat. No. 3,846,273 also includes a large proportion of a platinum group metal while the electrodes described in U.S. Pat. No. 4,422,917 include two layers: a thin layer of titanium dioxide mixed with niobium oxide or tantalum oxide plus a second layer which includes titanium dioxide with a large admixture of a platinum group metal. We specify that our electrodes not include an electrochemically significant amount of a platinum group metal for reasons recited in the previous paragraph.
U.S. Pat. No. 4,422,917 reports a ceramic electrode material of the general composition TiO.sub.x, where x may be in the range 1.55 to 1.95. In effect, these electrodes consist entirely of the oxide coat, with no metallic substrate. This material is produced by reducing titanium dioxide by heating it in an atmosphere of hydrogen at a temperature typically near 1150.degree. C. In this range of composition, the electrical conductivity of titanium oxide is sufficient to use the material as an electrode.
It is easy to deposit a layer of titanium dioxide upon the surface of a titanium metal substrate, or to produce a layer of titanium dioxide upon the surface of the metal by heating the metal in air at elevated temperature. In principle, the titanium dioxide layer thus deposited may be reduced to convert it to a composition TiO.sub.x where x is in the range 1.55 to 1.95 by heating the electrode in an atmosphere of hydrogen at 650.degree. C. or higher temperature. As noted in U.S. Pat. No. 4,422,917, this procedure causes severe hydriding and embrittlement of the titanium metal substrate, making the electrode useless. This observation has been confirmed by us. For this reason, an electrode with a titanium metal substrate cannot be prepared with a conductive coating of TiO.sub.x as described in U.S. Pat. No. 4,422,917.
Furthermore, TiO.sub.x electrodes are rapidly damaged and develop a low conductivity layer of TiO.sub.2 when operated at a potential sufficient to produce hydroxyl free radical (Graves and others, 1991), and therefore are impractical for this purpose.
Salvador (1980) described the preparation and properties of polycrystalline TiO.sub.2 electrodes doped with 0.12, 1.2 and 13.1 mole percent Nb. This material was prepared from TiO.sub.2 and Nb.sub.2 O.sub.5, by repeated grinding, pressing, and sintering. The sintering temperature was 1200.degree. C., and the cumulative sintering time approximately 50 hours under an atmosphere of nitrogen. X-ray diffraction indicated a solid solution with the rutile structure. The electrodes thus prepared were evaluated for possible use in a process to produce hydrogen from water utilizing solar energy.
Khodos and others (1988) described TiO.sub.2 doped with up to 13 mole percent Nb and its preparation by sintering at 1230.degree. to 1400.degree. C., and concluded that the material was a solid solution with the crystal structure of rutile. Babich and others (1988) reported a similar material produced by combining a solution of TiO.sub.2 in hydrochloric acid with a solution of Nb.sub.2 O.sub.5 in oxalic acid, adding sufficient ammonium hydroxide to the mixture to increase pH to 8.3 and precipitate a mixed hydrous oxide, and finally heating the washed precipitate. Neither Khodos nor Babich attempted to prepare electrodes from the material, and were primarily interested in describing the crystal structure and phase relations in the system TiO.sub.2 -Nb.sub.2 O.sub.5.
Madou and others (1984) described the properties of an electrode consisting of a large single crystal of rutile doped with 0.03 atom percent Nb, which was being evaluated as a possible pH sensor for high temperature use. For reasons of cost, single crystal electrodes are practically limited to applications where cost is of little importance, and the electrode need not be very large.
Salvador, Madou, Khodos and others, and Babich and others all considered the materials they had produced to be solid solutions of TiO.sub.2 and Nb.sub.2 O.sub.5 with the rutile structure. While prolonged heating at high temperature under vacuum or inert gas may cause some small degree of reduction of Nb.sup.+5 to Nb.sup.+4, and to a lesser degree reduction of Ti.sup.+4 to Ti.sup.+3, the materials described by the cited authors consisted predominantly of TiO.sub.2 and Nb.sub.2 O.sub.5.
Ammonium niobate is a substance whose variable composition is approximated by the formula NH.sub.4 NbO.sub.3 .multidot.xH.sub.2 O where x=0 to 2. The preferred embodiment of our process for producing the electrodes requires ammonium niobate as the source of niobium. Prior to our method for coating electrodes, there were no uses for ammonium niobate, and the ammonium niobate existed only as a subject of laboratory study. Ammonium niobate is not commercially available. Only laboratory methods for preparing ammonium niobate were found in the literature, and these methods are poorly suited for producing more than a few grams of ammonium niobate at a time, and are inefficient even on that small scale.
Ammonium niobate is highly soluble in water, and solutions including up to 21 w/w% Nb.sub.2 O.sub.5 may be prepared by carefully controlling the mole ratio NH.sub.4.sup.+ /Nb (Guerchais and Spinner, 1965). The solubility of ammonium niobate decreases rapidly with increasing concentration of NH.sub.4 OH (same ref.), and therefore, ammonium niobate may be precipitated from solution by adding ammonium hydroxide and ammonium salts.
Guerchais and Rohmer (1964) synthesized ammonium niobate by double displacement of potassium niobate (also very soluble in water) with ammonium chloride in a solution that also included ammonium hydroxide. Their procedure is the nearest prior art. Thirty grams of potassium niobate (stated by Guerchais and Rohmer to have the composition K.sub.7 HNb.sub.6 O.sub.19 .multidot.12H.sub.2 O) was dissolved in 1.2 L of water, then 300 mL of concentrated aqueous NH.sub.4 OH and 150 g of NH.sub.4 Cl were added, and the reaction mixture was cooled to 0.degree.-10.degree. C. Needle-like crystals of ammonium niobate subsequently formed in the solution. The reaction mixture described by them included 1.9N NH.sub.4 Cl, 2.9N NH.sub.4 OH, and 1.2 w/v% Nb.sub.2 O.sub.5. With this composition, the mixture would not precipitate ammonium niobate at room temperature. At T=0.degree.-10.degree. C., it is barely supersaturated, and ammonium niobate precipitates slowly and incompletely to form well-formed crystals, and a large fraction of the Nb remains in solution.
Also, the amount of concentrated NH.sub.4 OH and NH.sub.4 Cl used is large in relation to the amount of ammonium niobate produced. A practical industrial process would require that reagents be recovered from the reaction mixture and reused. In practice, ammonium hydroxide would be recovered by distillation, and ammonium chloride by sublimation. Ammonium chloride sublimes near 340.degree. C., a temperature that is inconveniently high for this purpose. The residue after sublimation would consist of byproduct KCl and Nb.sub.2 O.sub.5 not converted to ammonium niobate; the KCl would need to be dissolved in water and separated from the Nb.sub.2 O.sub.5 in order for the Nb.sub.2 O.sub.5 to be reused. Because of the large concentration of chloride ion in the reaction mixture, the ammonium niobate produced may be contaminated with ammonium chloride. For all of these reasons, the method described by Guerchais and Rohmer (1964) is inefficient in regard to utilization of reagents and product yield, and is poorly suited for industrial production and purification of ammonium niobate.